Policymakers at the highest level have recently called for broad increases in
instructional time to improve student achievement in the United States. Nevertheless,
due to disparities in methodologies and focuses, previous empirical research on the
effectiveness of expanding instructional time has produced largely mixed and
inconclusive results. This paper investigates the effects of changes in instructional hours
per day and days per year (and, by extension, the distribution of instructional time) on
eighth-grade student achievement using the international TIMSS Assessments. I
merged data from four quadrennial testing cycles (between 1995 and 2007) – covering
more than 800,000 student observations across seventy countries – to exploit the
tremendous variation in instructional time across countries and years.
This study finds that the relationship between student achievement and instructional
time follows a nonlinear, concave specification. In particular, I find that achievement
can be maximized at particular levels of instructional time – 181 days per year and 7
hours per day. Given that the levels of instructional time for the average school in the
United States appear to be near this optimum, the marginal effect of an extra day in the
U.S. is statistically insignificant, while an additional hour per day increases achievement
by only 0.011 standard deviations. I also find that these effects are heterogeneous for
different cohorts of students – namely, underprivileged students derive greater benefits
from increased instructional time than those who are more privileged. Thus, this study
provides evidence that an expansion of instructional time cannot be expected to
automatically increase student achievement, and extending the length of the school day
appears to be more effective than extending the length of the school year. Policymakers
would also be well advised to consider both the context and costs surrounding an
increase in instructional time.